Q29 of 43 Page 409

The work function for the following metals is given:

Na: 2.75 eV; K: 2.30 eV; Mo: 4.17 eV; Ni: 5.15 eV. Which of these metals will not give photoelectric emission for a radiation of wavelength 3300 Å from a He-Cd laser placed 1 m away from the photocell? What happens if the laser is brought nearer and placed 50 cm away?

Given:
























Element



Work function Φ0



Na



2.75 eV



K



2.30eV



Mo



4.17 eV



Ni



5.15 eV



Wavelength of radiation from laser = 3300 Å = 3300 × 10-10m


Energy radiated by the laser, E is given by,


…(1)


Where,


E = Energy of photon


h = Planck’s constant = 6.6 × 10-34 Js


c = 3 × 108 ms-1


λ = wavelength of photon


putting values in equation (1) we get,



E = 6 × 10-19 J


E = (6/1.6) × 10-19


E = 3.158 eV


We observe that Na and K have work functions less than the energy radiated by the laser so photoelectric effect will take place in case of them, whereas no photo-electric effect will occur in case of Mo and Ni.


Even if the laser is brought closer, it will have no effect on the outcome as the photo-electric effect is independent of the intensity of light.


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27

Monochromatic radiation of wavelength 640.2 nm (1nm = 10–9 m) from a neon lamp irradiates photosensitive material made of caesium on tungsten. The stopping voltage is measured to be 0.54 V. The source is replaced by an iron source and its 427.2 nm line irradiates the same photo-cell. Predict the new stopping voltage.

 28

A mercury lamp is a convenient source for studying frequency dependence of photoelectric emission since it gives a number of spectral lines ranging from the UV to the red end of the visible spectrum. In our experiment with rubidium photo-cell, the following lines from a mercury source were used:

λ1 = 3650 Å, λ2 = 4047 Å, λ3 = 4358 Å, λ4 = 5461 Å, λ5 = 6907 Å,


The stopping voltages, respectively, were measured to be:


V01 = 1.28 V, V02 = 0.95 V, V03 = 0.74 V, V04 = 0.16 V, V05 = 0 V


Determine the value of Planck’s constant h, the threshold frequency and work function for the material.


[Note: You will notice that to get h from the data, you will need to know e (which you can take to be 1.6 × 10–19 C). Experiments of this kind on Na, Li, K, etc. were performed by Millikan, who, using his own value of e (from the oil-drop experiment) confirmed Einstein’s photoelectric equation and at the same time gave an independent estimate of the value of h.]

 30

Light of intensity 10–5 W m–2 falls on a sodium photo-cell of surface area 2 cm2. Assuming that the top 5 layers of sodium absorb the incident energy, estimate time required for photoelectric emission in the wave-picture of radiation. The work function for the metal is given to be about 2 eV. What is the implication of your answer?

 31

Crystal diffraction experiments can be performed using X-rays, or electrons accelerated through appropriate voltage. Which probe has greater energy? (For quantitative comparison, take the wavelength of the probe equal to 1 Å, which is of the order of inter-atomic spacing in the lattice) (me = 9.11 × 10–31 kg).